Many electronic ballasts for powering gas discharge lamps include a driven half-bridge inverter and a series resonant output circuit. Such ballasts generally include some form of protection circuitry for preventing damage to the inverter and other portions of the ballast in the event of a lamp fault condition. Common lamp fault conditions include lamp removal or lamp failure.
A popular protection approach is to place a current-sensing resistor in series with the lower inverter transistor, monitor the voltage across the current-sensing resistor, and shut down the inverter if the voltage across the current-sensing resistor exceeds a predetermined threshold value. While this approach is adequate for protecting against certain fault conditions, such as lamp removal or lamp failure, it does not adequately protect against less well-defined fault conditions, such as the arcing that occurs when a slight air gap is introduced between the pins of a lamp and the sockets of the lighting fixture. Under such an emergent arcing situation, the voltage that develops across the current-sensing resistor will not necessarily be high enough to exceed the predetermined threshold value, in which case the inverter will continue to operate and the potentially dangerous arcing condition will be allowed to continue unabated.
Simply lowering the resistance of the current-sensing resistor (and, thus, the predetermined threshold value) is not a successful remedy to this problem, because that might result in the inverter being improperly shut down even in the absence of a legitimate fault condition. This is especially true for ballasts that must be capable of powering several different types of lamps (e.g., F17T8, F25T8, and F32T8 lamps), in which case the current that flows through the current-sensing resistor during normal operation (i.e., with no fault condition present) may vary over a considerable range. Thus, in order to avoid false detection of a fault, the predetermined threshold value must be set such that the current through the current-sensing resistor must be much higher than the normal operating value before a fault is detected. Of course, when a mild arcing condition occurs, the current that flows through the current-sensing resistor may increase only modestly above its normal operating value, in which case the predetermined fault threshold will not be reached and the inverter be allowed to continue to operate.
What is needed, therefore, is a ballast with a fault detection circuit that is capable of quickly and accurately responding to an arcing condition in the lamp load. Such a ballast would represent a significant advance over the prior art.